Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210914171551.3223715-1-pgonda@google.com> <YUDcvRB3/QOXSi8H@google.com>
In-Reply-To: <YUDcvRB3/QOXSi8H@google.com>
From:   Peter Gonda <pgonda@google.com>
Date:   Tue, 14 Sep 2021 11:58:37 -0600
Message-ID: <CAMkAt6opZoFfW_DiyJUREBAtd8503C6j+ZbjS9YL3z+bhqHR8Q@mail.gmail.com>
Subject: Re: [PATCH] KVM: SEV: Disable KVM_CAP_VM_COPY_ENC_CONTEXT_FROM for SEV-ES
To:     Sean Christopherson <seanjc@google.com>
Cc:     kvm list <kvm@vger.kernel.org>, Marc Orr <marcorr@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Nathan Tempelman <natet@google.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Sep 14, 2021 at 11:32 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Sep 14, 2021, Peter Gonda wrote:
> > Copying an ASID into new vCPUs will not work for SEV-ES since the vCPUs
> > VMSAs need to be setup and measured before SEV_LAUNCH_FINISH. Return an
> > error if a users tries to KVM_CAP_VM_COPY_ENC_CONTEXT_FROM from an
> > SEV-ES guest.
>
> What happens if userspace does KVM_CAP_VM_COPY_ENC_CONTEXT_FROM before the source
> has created vCPUs, i.e. before it has done SEV_LAUNCH_FINISH?

That's not enough. If you wanted to be able to mirror SEV-ES you'd
also need to call LAUNCH_UPDATE_VMSA on the mirror's vCPUs before
SEV_LAUNCH_FINISH. That is do-able but I was writing a small change to
fix this bug. If mirroring of SEV-ES is wanted it's a much bigger
change.

>
> Might be worth noting that the destination cannot be an SEV guest, and therefore
> can't be an SEV-ES guest either.

sev_guest() implies sev_es_guest() so I think this case is covered.

>
> > Fixes: 54526d1fd593 ("KVM: x86: Support KVM VMs sharing SEV context")
>
> Cc: stable@vger.kernel.org

Oops. I'll update in the V2 if needed. Added to this thread for now.